Elemental distributions and mineral parageneses of the Skaergaard PGE-Au mineralisation: consequences of accumulation, redistribution, and equilibration in an upward-migrating mush zone

This is the author accepted manuscript. The final version is available from Oxford University Press via the DOI in this recordThe Skaergaard PGE-Au mineralisation, aka the Platinova Reef, is a syn-magmatic Platinum Group Element (PGE) and gold (Au) mineralisation that formed after crystallisation of ∼74% of the bulk melt of the intrusion. It is hosted in a more than 600 m deep and bowl-shaped succession of gabbroic macro-rhythmic layers in the upper 100 m of the Middle Zone. The precious metal mineralisation comprises a series of concordant, but compositionally zoned, mineralisation levels identified by distinct PGE, Au and Cu peaks. They formed due to local sulphide saturation in stratiform concentrations of interstitial and evolved mush melts in six MLs over > 2000 years. The PGE-Au mineralisation is compared to a stack of gold-rimmed saucers of PGE-rich gabbro of upward decreasing size. Fundamentally different crystallisation and mineralisation scenarios have been proposed for the mineralisation, including offset reef type models based on sulphide saturation in the melt from which the silicate host crystallised, and the here argued model which restricts the same processes to the melt of the inward migrating mush zone of the magma chamber. The latter is supported by: i) a 3D summary of the parageneses of precious metal minerals and phases (> 4000 grains) from 32 samples across the mineralisation; ii) a 3D compilation of all bulk rock assay data; and iii) a principal component analysis (PCA) of PGE, Au, Cu, and selected major and trace elements. In the main PGE-mineralisation level (Pd5 alias Pd-Zone) the precious metal mineral paragenesis varies across the intrusion with precious metal sulphides and Au-alloys at the W-margin to Precambrian basement, precious metal plumbide and Au- and Ag alloys at the E-margin to flood basalts, and skaergaardite (PdCu) and intermetallic compounds and alloys of PGE-Au and Cu in the central parts of the mineralisation. Precious metal parageneses are distinct for a given sector of the intrusion, i.e., drill core (local control), rather than for a given stratigraphic or temporal interval in the accumulated gabbros. The precious metal “grade times width” number (average g/t x metres) for the mineralisation at an upper and a lower cut off of 100 ppb PGE or Au increases from ∼20 to ∼45 g toward the centre of the mineralisation due to ponding of precious metal bearing melt. A strong increase in (Pd+Pt+Au)/Cu and dominance of (PdCu) alloys in the lower and central parts of the mineralisation demonstrate the partial dissolution of droplets of Cu-rich sulphide melt and fractionation of precious metal ratios. The precious metal parageneses, the distribution of precious metals in the mineralisation, and the PCA support initial accumulation of precious metals in the melt of the mush in the floor, followed by equilibration, sulphide saturation, and reactions with residual and immiscible Fe-rich silicate melt in a series of macro-rhythmic layers in the stratified and upward migrating mush zone in the floor of the magma chamber. Syn-magmatic and upward redistribution of precious metals sets the Skaergaard PGE-Au Mineralisation apart from conventional reef type and offset-reef type precious metal mineralisations, and characterize “Skaergaard type” precious metal deposits.Geological Survey of Denmark and Greenlan

Extreme enrichment of Se, Te, PGE and Au in Cu sulfide microdroplets: evidence from LA-ICP-MS analysis of sulfides in the Skaergaard Intrusion, east Greenland

The Platinova Reef, in the Skaergaard Intrusion, east Greenland, is an example of a magmatic Cu–PGE–Au sulfide deposit formed in the latter stages of magmatic differentiation. As is characteristic with such deposits, it contains a low volume of sulfide, displays peak metal offsets and is Cu rich but Ni poor. However, even for such deposits, the Platinova Reef contains extremely low volumes of sulfide and the highest Pd and Au tenor sulfides of any magmatic ore deposit. Here, we present the first LA-ICP-MS analyses of sulfide microdroplets from the Platinova Reef, which show that they have the highest Se concentrations (up to 1200 ppm) and lowest S/Se ratios (190–700) of any known magmatic sulfide deposit and have significant Te enrichment. In addition, where sulfide volume increases, there is a change from high Pd-tenor microdroplets trapped in situ to larger, low tenor sulfides. The transition between these two sulfide regimes is marked by sharp peaks in Au, and then Te concentration, followed by a wider peak in Se, which gradually decreases with height. Mineralogical evidence implies that there is no significant post-magmatic hydrothermal S loss and that the metal profiles are essentially a function of magmatic processes. We propose that to generate these extreme precious and semimetal contents, the sulfides must have formed from an anomalously metal-rich package of magma, possibly formed via the dissolution of a previously PGE-enriched sulfide. Other processes such as kinetic diffusion may have also occurred alongside this to produce the ultra-high tenors. The characteristic metal offset pattern observed is largely controlled by partitioning effects, producing offset peaks in the order Pt+Pd>Au>Te>Se>Cu that are entirely consistent with published D values. This study confirms that extreme enrichment in sulfide droplets can occur in closed-system layered intrusions in situ, but this will characteristically form ore deposits that are so low in sulfide that they do not conform to conventional deposit models for Cu–Ni–PGE sulfides which require very high R factors, and settling of sulfide liquids

Separation of accessory minerals from rocks and ores by hydroseparation (HS) technology: method and application to CHR-2 chromitite, Niquelandia intrusion, Brazil

The hydroseparator embodies novel technology that can be of great benefit for the study of accessory minerals in rocks, ores, industrial products and materials of potential environmental concern. The technique is described and an example is provided of its application in the study of the platinum-bearing CHR-2 chromitite of the Niquelandia layered intrusion, central Goias, Brazil